The present invention relates to the technical field for the production nonwovens with the aid of a carding machine. In this particular field, one of the main objects of the invention is to provide a method and installation for producing a nonwoven whose fibres have been re-orientated, said nonwoven being commonly designated as a condensed nonwoven. The invention can be applied in particular, not but exclusively, for the production of synthetic (polypropylene, polyethylene, etc) fibre-based condensed nonwovens having a high weight, and a weight typically greater than 80 g/m2.
Nonwovens produced by means of a carding machine can be classified in two categories: xe2x80x9cparallelxe2x80x9d nonwovens whose fibres are essentially orientated parallel to one another in the machine direction (carding direction); and so-called condensed nonwoven whose fibres have undergone a re-orientation.
In practice, parallel nonwovens are nonwovens whose mechanical properties exhibit an extremely low isotropy with an extremely low mechanical resistance in a transverse direction compared to the machine direction. On the other hand, the condensed nonwovens have from a mechanical point of view a better isotropy owing to the re-orientation of their fibres. In practice, condensed nonwovens have a higher weight (g/m2) and are more mechanically resistant in the transverse direction compared with the parallel nonwovens.
Usually, so as to produce a condensed nonwoven, a doffer cylinder and at least one first condenser cylinder are used at the outlet of the carding machine. The said doffer cylinder is juxtaposed at the main cylinder of the carding machine (still commonly referred as the xe2x80x9cgrand tambourxe2x80x9d), and the said first condenser cylinder is juxtaposed to the doffer cylinder. The doffer cylinder is a cylinder having on its periphery a specific clothing whose teeth are orientated in a direction opposite its direction of rotation. said clothing having the functionality to remove one portion of the individual fibres from the main cylinder of the carding machine and to card these fibres so as to render them parallel. The condenser cylinder is also provided on its periphery with a specific clothing whose teeth are orientated in a direction opposite its direction of rotation, said condenser cylinder being rotated in a direction opposite the rotating direction of the doffer and with a clearly slower circumferential speed. This difference of speeds allows a loading of the clothing of the condenser cylinder with the parallel fibres issued from the doffer cylinder, said fibres in addition being re-orientated (or mixed up) at the time they are transferred between the two cylinders. This re-orientation is explained by the fact that, when transferred, the fibres are upturned by being retained at their front portion by the clothing of the condenser cylinder driven at a slow speed and by being entrained at their rear portion at higher speed by the clothing of the doffer cylinder. So as to increase the weight per square metre of the condensed nonwoven, it is possible to provide a second condenser cylinder juxtaposed to the first condenser cylinder and driven with a slower speed.
The condensed nonwoven obtained at the outlet of the carding machine, that is to say the condensed nonwoven obtained at the outlet of the last condenser cylinder of the carding machine, is then transported by any known device to a consolidation station which is selected according to the targeted application for the nonwoven (binding of fibres by water jets, calendering, needling, etc).
The difference of speeds between the condenser cylinder and the preceding cylinder directly conditions the weight of the nonwoven obtained. However, it has been observed in practice that this difference of speeds cannot be too great as beyond a certain limit, waves are created in nonwoven when it passes from one cylinder clothing to another, which results in deteriorating the quality of the condensed nonwoven produced at the carding machine outlet. This limitation is expressed by a limitation concerning the weight (per square meter) of the non woven produced. Generally speaking, by optimising the doffer cylinders and condensers cylinder of the carding machine, it is possible to obtain synthetic fibre-based condensed nonwoven of the polypropylene type having at the most a weight per square meter between 60 g/m2 and 80 g/m2.
Amongst other technical solutions for producing a condensed nonwoven, it has already been recommended to replace, at the outlet of the carding machine, the previously described conventional condenser cylinder with a condenser element without any clothing on its periphery. This type of technical solution is described for example in patent application GB-A-962162 or in U.S. Pat. No. 3,787,930.
In patent application GB-A-962162, it is more particularly taught to implement a smooth suction condenser drum which picks up the fibres directly from the periphery of the main drum of the carding machine (without any intermediate doffer cylinder). The picking up of the fibres from the periphery of the carding machine drum is effected via the suction of the condenser drum. In one preferred embodiment variant, so as to help in taking off the fibres retained at the periphery of the main drum of the carding machine, an additional flow of air is provided in the form of a jet of air blown in the direction of the teeth or equivalent of the clothing of the main drum.
In U.S. Pat. No. 3,787,930, a condensed nonwoven is formed with the aid of two belt conveyors adjacent to the doffer cylinder of the carding machine and a suction of fibres in the transfer zone between the doffer cylinder and the belt conveyors.
Firstly, to the knowledge of the Applicant, the solutions described in said publications GB-A-962162 and U.S. Pat. No. 3,787,930 have never been industrially exploited. Secondly, as these solutions are comparable to the conventional solution with a condenser cylinder equipped with a peripheral clothing, the nonwoven undergoes condensation at the time it is formed from fibres picked up from an upstream cylinder, also provided with a peripheral clothing, it can be assumed that the same limitations will be encountered with these solutions as for the conventional solution in terms of differences of speed and thus in terms of weight of the produced condensed nonwoven.
The present invention seeks to provide a new method for the production of a condensed nonwoven.
According to this method, at least a first one nonwoven is conventionally produced with the aid of a carding machine.
According to the invention, said first nonwoven web is made to undergo a condensing operation. To this effect, said first nonwoven issued from the carding machine is transported to a rotating transfer element by means of a first transport surface. Said first nonwoven is transferred onto a second transport surface by means of said transfer element, the second transport surface being driven at a linear speed slower than the circumferential speed of the transfer element. The speed differential between the transfer cylinder and the second transport surface allows an accumulation of fibres in the transport zone between these two elements and thus a re-orientation of the fibres of the first nonwoven. Thus, from this first nonwoven, a condensed nonwoven is formed having a weight per square meter greater than the weight per square meter of the first nonwoven issued from the carding machine.
In the method of the invention and contrary to the case of aforesaid solutions of the prior art, the condensing operation is not effected during formation of the nonwoven but at a subsequent stage on a nonwoven already formed and derived from a carding machine. It has been observed that this condensing operation carried out at a subsequent stage on a nonwoven already formed would surprisingly make it possible to obtain a condensing rate much higher than that obtained with a conventional solution without adversely affecting the quality of the produced nonwoven.
Another object of the invention is to provide a condenser device for condensing nonwovens. This device comprises first and second transport surfaces and a rotating transfer element which during operation is able to transfer a nonwoven from the first to the second transport surface; during operation, the linear speed of the second transport surface is slower than the circumferential speed of the transfer element.
Another object of the invention is to provide an installation for producing a condensed nonwoven, said installation comprising a carding machine able to produce at least one first nonwoven and the aforesaid condenser device. The carding machine and the condenser device are arranged so that said first nonwoven produced by the carding machine is placed at the carding machine outlet on the first transport surface of the condenser device.